MONGOID-5935 Fixed UnknownAttribute error with nested attributes

[jamis]

This fixes the raised Mongoid::Errors::UnknownAttribute error when nested attributes include _destroy: true for a child whose _id is not already in the parent's relation (e.g. a new parent receiving an existing child's _id).

[Copilot]

Pull request overview

Fixes Mongoid::Errors::UnknownAttribute raised during nested-attributes processing when _destroy is provided for an existing child document that is not currently part of the parent’s relation (notably when reparenting via nested attributes is enabled).

Changes:

• Update nested many-association handling to skip reparenting/update logic when the incoming nested attributes are destroy requests (_destroy), preventing _destroy from being passed into update_attributes.
• Add regression specs for both has_many and has_and_belongs_to_many relations ensuring _destroy on a non-related existing document does not raise and does not add the document to the relation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/mongoid/association/nested/many.rb	Prevents _destroy payloads from flowing into reparenting/update paths for non-related existing documents.
spec/mongoid/attributes/nested_spec.rb	Adds coverage for destroy-on-non-related-document cases for has_many and HABTM nested attributes.

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